Circuit for generating pulses having steep wave fronts



J. P. LAMB Sept. 29, 1964 CIRCUIT FOR GENERATING PULSES HAVING STEEPWAVE FRONTS 6 Sheets-Sheet 1 Original Filed Dec. 31, 1956 lll-lINVENTOR. JEF/ZfiSdM P.

Sept. 29, 1964 J. P. LAMB 3,151,298

CIRCUIT FOR GENERATING PULSES HAVING STEEP WAVE FRONTS Original FiledDec. 31, 1956 6 Sheets-Sheet 2 i L W765! WWI] INVENTOR. JZF/EfiSd/V F.44/445 ZZZ/G. 8a.

.1. P. LAMB Sept. 29, 1964 CIRCUIT FOR GENERATING PULSE-S HAVING STEEPWAVE FRONTS Original F'iled Dec. 31, 1956 6 Sheets-Sheet 3 INVENTOR.Jf/FA'KSO/V 419MB Sept. 29, 1964 J. P. LAMB 3,

CIRCUIT FOR GENERATING PULSES HAVING STEEP WAVE FRONTS Original FiledDec. 51, 1956 6 Sheets-Sheet 4 5&0 1 ,2 5/4 I NVEN TOR. JZ/FEEJO/V 24,4445

J. P. LAMB Sept. 29, 1964 CIRCUIT FOR GENERATING PULSES HAVING STEEPWAVE FRONTS 6 Sheets-Sheet 5 Original Filed Dec. 31, 1956 L J ZINVENTOR. JZF/ZEfU/V P. 3445 6 Sheets-Sheet 6 J. P. LAMB One J'fyna/my gc/e I l l l lead 168 Mad/a0 Sept. 29, 1964 CIRCUIT FOR GENERATING PULSESHAVING STEEP WAVE FRONTS Original Filed Dec.

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Lead/61 United States Patent Office Ejblfidd Patented Sept. 22 F864 3151 298 crncorr iron. csrerlrmrmc PULSES navnro WAVE llibtll liltl.letferson ll. Lamb, Tulsa, (Elder, assignor to Dresser industries,lino, Dallas, Tex., a corporation of Delaware @riginal application Dec,ill, H56, No. 631,739.

Divided and this application Nov. 17, 1958, tier. Nu.

l tlllaims. (Cl. sas es) The invention hereinafter disclosed relates tomeans and methods of securing information relating to physicalcharacteristics of earth formations penetrated by a borehole such as awell; and more particularly the invention relates to an inventigativesystem of that class commonly termed electrical borehole loggingsystems.

This application is a division of an application of lefferson l. Lamb,Serial No. 631,789 filed December 31, 1956, entitled Electrical LoggingSystems for Earth Baseholes, now abandoned.

As ordinarily constituted, an electrical logging system for earthboreholes comprises means for supplying electric current to one or moreelectrodes so situated in the borehole that the current follows pathsthrough the borehole encircling earth to a return electrode, one or morepotential or pick-up electrodes subjected to electric field potentialscreated in the vicinity of the borehole by the electric current, andmeans for recording representations of the field potentials. Urdinarilythe current-emanating electrode system and the pick-up electrodes aredisposed on a tool or means traversable along the borehole by anelectric conductor cable which is payed out into and withdrawn from theborehole by winch apparatus. To avoid polarizing effects at theelectrode surfaces, the current supplied for passage through the earthis alternating in character.

To provide a more complete series of data in the record produced as theelectrode system is traversed along the borehole, it is desirable tosecure and record substantially continuous measures of the potentialscreated between electrodes arranged with different spacings anddilferent locations relative to the current electrode system. Forexample, it is desirable to secure records of the potentials exhibitedbetween the two (pick-up) electrodes under each of different pick-upelectrodes pair spacings, with current injected into theborehole-encircling formation at appropriate current electrode locationsspaced from the pick-up electrodes, to provide, for example, what are inthe art commonly termed short normal, long normal, short lateral, andlong lateral electrical logs or curves. In addition it is desirable tosecure and record indications of the variations in a DC. potentialexisting between spaced-apart electrodes in the borehole, and variouslyknown as the spontaneous potential or natural potential. These are, ofcourse, only exemplary of the types of information which it may bedesirable to log.

Various means and modes of supplying current to a current-emanatingelectrode system, and for measuring and recording the natural potential(hereinafter denoted NP.) and the pick-up electrode potentials, havebeen employed. in one known electrical logging system, current suppliedfrom generating means outside the borehole is conducted to thetraversing electrode-supporting tool in the borehole by way of asingle-conductor insulated el c trical cable, the current returning tothe generator by way of an earth path and a surface ground electrode orby way of a conductive armor sheath comprised in the cable. In thatsystem, the potentials are measured by apparatus contained in theborehole traversing tool, and the mensuration data communicated to therecording cation outside the borehole by frequency modulationtelemetering, using several high-frequency carrier waves. In anotherknown system of electrical logging, use is made of a multi-conductorinsulated electric cable comprising six or more individual conductorseach of which is devoted to continuous transmission of a single current(or voltage). The current for passage through the earth formations isconducted from an alternating current generator to the current electrodesystem in the borehole through one of the insulated conductors of thecable; and the potentials between the potential electrodes are conductedover others of the cable conductors to a location outside the boreholefor measurement and registration by recording means.

In all of the aforementioned electric. borehole-logging systems,considerable difficulty is experienced in attempting to secure accuraterepresentation of the mensuration data whose origin is at variousstations along the borehole. The principal difficulty is presented bythe fact that the intelligence transmission medium, namely the cableconductor or conductors, has varying and unpredictable transmissioncharacteristics as it is payed out from or reeled in by the winch and asa varying portion of the total length of cable is traversed through theborehole where temperatures, pressures and other factors affecting theelectrical characteristics of the cable are far from constant. As iswell known, resistance of any economically feasible cable conductorvaries with variations in temperature, so the resistances of theindividual cable conductors are continually varying in a non-linearmanner as the logging tool and cable move through zones of differenttemperatures during traverse along a borehole. It is also well knownthat inter-conductor capacitance varies as the cable conductors arereeled and unreeled and pass through or along different sections of theborehole, and that inter'conductor leakage resistance varies with ageand operating conditions. Another difticulty encountered in the attemptto secure accurate representations outside the borehole of the currentsand/or potentials existing at the electrode locations in the borehole,is presented by the so-called crosstalk which is manifest whenalternating or varying currents flow in conductors which are in closeproximity to each other. The conductors of the cable, situated in closeside-by-side relationship inside the cable sheath or armor, are markedlysubject to such cross-talk effects, any varying current in one of theconductors inevitably causing an undesired concurrent potential orcurrent to be manifest in each of the other conductors.

As may be deduced from the preceding brief sketch of adverseenvironmental and constructional factors faced in electrical loggingsystem design and practice, the systems heretofore employed weredeficient or ob-- jectionable in one or more respects in the matter ofproviding accurate graphical representations or logs of a plurality ofphysical characteristics of borehole-encircling earth. The frequencymodulated carrier Wave telemetering system requires a large amount ofelectronic equipment in the logging tool and hence suffers from all thedeficiencies and troubles resulting from operating that type ofequipment in a high-temperature environment where circuit elements tendto fail to maintain constant electrical characteristic values.Additionally, signal distortion and depreciation due to necessarychannel filter circuitry are undesirable features of that system; not tomention the voluminous nature or amount of apparatus necessary in thelogging tool traversed along the borehole. That logging system wherein alarge manyconductor cable is employed suffers from the drawback ofexcessive distortion or lack of precision in logs secured, because ofthe complex and very variable degree 3 of cross-talk and leakage betweenconductors; and also any cable of more than three insulated conductorsand sheath is initially a very expensive device of short useful life,and a component requiring an excessive amount of maintenance.

It has been demonstrated that of all plural-conductor armored cables,that type consisting essentially of three insulated electricalconductors and a conductive armor sheath of a plurality of spirallyapplied outer wires, provides the most durable and trouble-freeconstruction. Such a cable provides, in its sheath, and armor and aneffective return or ground conductor; and an individual conductor fortransmission of very low frequency N.P. variations and relatively highfrequency alternating current power, and two conductors which may beemployed for control and information signaling and other subsidiaryfunctions. The term three-conductor cable as hereinafter employed isused to define or designate a cable of the construction just describedand consisting essentially of three individually insulated electricalconductor units (either stranded or single wire, for example),insulation and an outer electrically conductive sheath or armor.

With the aforementioned deficiencies of prior electricalborehole-logging systems in view, it is a prime object of the presentinvention to provide an improved electrical borehole-logging system.Another object is to provide an electrical borehole-logging system freeof the deficiencies of frequency-modulation telemetering of logging dataand free of the deficiencies and objectionable features of loggingsystems employing many-conductor cable means in the borehole. Anotherprime object of the invention is to provide a constant-currentelectrical borehole-logging system. Another object is to provide anelectrical borehole-logging system utilizing a three-conductor cable andcapable of providing at least five accurate logs for each traverse alonga borehole. Another object is to provide a logging system utilizing athree-conductor cable and capable, during each borehole traverse, ofproviding short and long normal electrical logs, short and long lateralelectrical logs, and an N.P. log.

Another object of the invention is to provide an effective electricallogging system for producing a plurality of logs at each loggingtraverse with a minimum number of subsurface electron tube devices.

Another object of the invention is to provide means whereby electricpower for simultaneously operating subsurface apparatus and emanatingconstant current from the subsurface current-emanating electrodes istransmitted through a single conductor and sheath of a plural-conductorcable.

Another object of the invention is to provide an electrical loggingsystem with means for calibrating subsurface apparatus during a loggingtraverse of an earth borehole.

Another object of the invention is to provide an improved means andmethod for signaling logging information ordata from within an earthborehole during a logging traverse.

Another object of the invention is to provide an improved means and modefor producing an electric control pulse having an extremely sharp wavefront, from a grossly distorted direct current pulse of much greaterduration.

Another object of the invention is to provide an improved means and modefor providing alternating current of constant intensity for transmissionto the current-electrode system and other apparatus of a logging tool.

Another object of the invention is to provide a logging system in whichthe adverse effects of inter-conductor leakage of alternating currentare nullifiied or rendered of no consequence.

Another object is to provide a novel means and mode for regulating aconstant current A.C. generator whereby phase-shift in the outputcurrent does not produce a change in intensity of the generator output.

Another object is to provide a novel logging system constant-currentA.C. generator with instantaneous regulation of output.

Another object is to provide a logging system having means whereby thepositions of switch means in a logging tool may be changed by remotelycontrolled means in surface apparatus, and the positional status of theswitch means there ascertained and indicated.

Another object of the invention is to provide a novel alternatingcurrent generator having substantially instantaneous regulation of itsoutput.

Another object is to provide an improved pulse-formation circuit.

An additional feature and object of the invention is the provision ofmeans whereby calibration of subsurface instrumentalities in the loggingtool may be eifected quickly and during a borehole traverse, whereby theoperator may quickly determine the operation-characteristics of thesubsurface apparatus.

The aforementioned and other objects and advantages hereinafter madeapparent are accomplished by the invention, a preferred exemplaryembodiment of apparatus conforming to the principles thereof beingdiagrammatically depicted in the accompanying drawings, in which:

FIG. 1 is a schematic diagram depicting the earth borehole environmentin which the invention is practiced, with principal surface andsubsurface apparatus components indicated and illustrated principally inthe form of a functional block diagram;

FIGS. 2a and 2b comprise a schematic circuit diagram of that portion ofthe surface apparatus, at the operating location outside the borehole,which is included in the block diagram enclosed by the dot-dash line inthe upper part of FIG. 1;

FIGS. 3a and 31) comprise a schematic circuit diagram of the principalcomponents of the subsurface apparatus which is principally enclosed inthe logging tool adapted to be traversed along an earth borehole;

FIGS. 4 and 4a are graphical representations on Cartesian coordinates ofcurrent-voltage characteristics of certain components of the constantcurrent supply unit of the exemplary apparatus;

FIG. 5 is a series of graphical representations ofresistivity-representing signals;

FIGS. 6 and 6a are graphical representations of potentials occurring atcertain locations in the surface apparatus;

FIG. 7 is a series of related graphical representations of potentials orwaves, relating to signal gating operations;

FIG. 8 is a circuit diagram of certain switch and power circuit meansemployed in performing subsurface switching1 operations andswitch-position indicating operations; an

FIG. 9 is a portion of the diagram of FIG. 2b with an added element formodifying the circuit operation.

FIGS. 2a, 2b, 3a and 3b in composite illustrate in schematic form thesubsurface apparatus situated at the operating location outside an earthborehole, the subsur face apparatus which in operation is traversedalong an extent of borehole, and the cable means by which the subsurfaceapparatus or tool is supported and traversed along the borehole andwhich concurrently conveys power from the surface apparatus to thesubsurface and serves as a signal transmission medium interconnectingthe surface and subsurface apparatuses. Due to the considerations andenvironmental and operational factors hereinbefore briefly mentioned,the cable is restricted to three insulated conductors and a conductivesheath or armor. Except in respect of certain details hereinafter madefully evident, the actual physical assembly and mounting of theelectrical and electronic components of the surface and subsurfaceapparatuses may vary Widely in design, it being obvious that thesubsurface apparatus must be suitably housed in an elongated vessel orcontainer adapted to withstand the very high external pressures and hightemperatures encountered in deep liquidfilled boreholes. Further, as isevident to those skilled in the art, the mentioned vessel must be ofsuitably small transverse dimensions to permit of operation in earthboreholes, and must be fluid-tight.

Since the exemplary logging system of the invention requires emanationof current of constant intensity from the current electrodes of thesubsurface apparatus or tool, and since this current is alternating incharacter and is sup plied through the variable resistance of the cable,an alternating current generator capable of supplying constant currentunder variable load conditions is necessary. The current is commutatedin the logging tool and to different sets of current electrodes, eachset in its turn. Utilization of a current of constant intensity permitsmeas urement of only voltage rather than both current and voltage,thereby considerably reducing the amount of apparatus needed, andpresents the additional marked advantage of eliminating the effects ofvarying resistance of the current circuit during borehole traverse. Theaccuracy of the logs or graphical records of the resistivity indicationsor measures is, therefore, strictly dependent upon constancy of theintensity of the alternating current employed at the current electrodes,and hence it is doubly necessary that close automatic control of thegenerator be maintained. A novel means for this function is provided bythe invention. As will become evident, by utilizing direct currentpulses for signaling the resistivity logs information from subsurface tothe surface, and by eliminating at the surface the distorted leading andtrailing edges of those DC. pulses, an extremely high degree of accuracyor fidelity in the information signaling is attained, as compared withany system wherein alternating current waves are employed for thetransmission of valuerepresenting signals. This improvement in accuracyin the information-signaling is attained largely because of the steadystate condition of the signal circuits during the middle portion of eachdirect current signal pulse period. Since the effects of all transientcurrents, cross-talk, leakage currents, and other varying or alternatingcurrents may readily be attenuated to extinction during the selectedmiddle portion of the signal pulse period, and without adverse effectupon the middle portions of the signal pulses, a truly accuratetelemetering of the samples of information can be attained. And sincethe commutation rate may be so chosen, with respect to the speed of theelectrode system along the borehole, that a sample of information foreach of the several resistivity information channels is secured for eachof small increments of borehole traverse, the fidelity of the producedgraphic logs or records may be as high as may be desired. As an example,with a commutator shaft rotation speed of 500 revolutions per minute inan exemplary system providing four information logs, and a boreholetraversal speed of one hundred feet per minute, a sample of information(resistivity measurement, for example) is secured for each informationchannel or log each 2.4 inches along the traversed extent of borehole.That final log produced from such samples is truly representative forall practical purposes is readily apparent.

An exemplary system according to the invention may be briefly outlinedwith reference to FIG. 1, Which depicts in diagrammatic form the basicorganization and environment of an exemplary embodiment of apparatus ina system for providing four similar logs and an NP. log. A fluid-filledborehole Biz extends downwardly through earth formations to be logged. Acable Ca extends into the borehole and therein supports for traverse thelogging tool Lt containing the subsurface apparatus. The latterapparatus comprises components indicated diagrammatically within thelower part of FIG. 1 adjacent 0 iii) and to the right of the loggingtool Lt. The cable extends over a guide sheave Pu and is paid out fromand Wound upon a Winch drum Wd, as Well known in the art; and the cablesheath conductor 3 and insulated cable conductors l, 2, and 3 areconnected through conventional slip ring and brush means Sr to leads 8',l and 2. and 3' of the surface apparatus. The latter apparatus comprisescomponents enclosed Wi hin the dash-dot line enclosure Constantintensityalternating current is supplied by a power supply unit 5 through aconverter drive unit s and DC. blocking capacitor Cl, to respectiveleads I. and S for transmission to and from the subsurface apparatus. inthe latter, the alternating current path is from conductor ll, through aDC. blocking capacitor C2, the pri maries of transformers Till and Tilt,through the motor of a shaft driving unit 7, and a current: commutatorunit Cc. From the latter the current is commutatively passed cyclicallyin discrete pulses through switch units Swll, SW23, SW3, SW4 torespective current electrodes Be, Be, E7 and Hg, the current thenreturning to the surface by Way of the earth and a remote ground to thegenerator. The remote ground may be an exposed portion of the cablesheath forming conductor S, it being understood that a certain lower-endportion Sr of the cable sheath may be covered with insulation to provideremoteness for the ground connection with respect to the electrodes.

Passage of the pulses of alternating current through respectiveearth-paths creates alternating electric potential fields, within whichare situated pick-up electrodes such as Ea, Ed suitably positioned onthe logging tool LI (but indicated at the right of Pi l), and exposed todetect the field potentials at their respective locations. Thepotentials thus sensed or picked-up are preferably taken with referenceto a ground electrode inserted into the earth in the vicinity of thesurface apparatus. That ground electrode will hereinafter be termed asurface ground, as distinguished from the cable sheath. The sensedpotentials, (four in each signaling cycle in the exemplary embodiment ofthe invention), may diii'er widely in intensity due to diiferences inthe current electrode and pick-up electrode configurations; and hencethe sensed potentials are differently treated prior to formation offield potential-representing signals by the subsurface apparatus. Thepotentials sensed at Eat and Ed are so individually attenuated that asingle amplifying means to which they are each in turn presented, mayeasily ac cornmodate the differences in intensity of potential withoutsignificant distortion. Actually each of the potentials sensed atelectrodes Ed and En (two potentials between Ed and ground and twobetween Ea and Ed) exists for a short period only, the periods ofexistence being determined by the periodic flows of alternating currentthrough the respective current electrodes Ea, Ec, Hg and E Thus tWofields are sampled, one following the other by a short interval, betweenEd and ground; and then two fields are similarly sampled between Ed andEn, in a manner hereinafter more fully explained. The four fieldpotential samples secured are passed through respective switch means ll,l2, 13 or thence through respective attenuator means 15, l6, l7 or E8,and thence into respective signal channel isolating means 2t, 22, 23 or24. The isolation means are employed to feed the time-spaced potentialsignals of the first, second, third and fourth signal channels justmentioned into a common signal amplifier, each signal in its turn on atime-division basis. Also the isolation means serves to isolate anatural potential of continuous nature exhibited at the electrodes, fromthe A.Q. signal channels.

The signals representing the field potential samples corresponding tothe four electrode configurations mentioned, and which are produced inresponse to commutation of electrode current by commutator Cc, are eachin the form of a short burst of AC. when fed through the respectiveisolation means 21, 2-2, etc. The signals are produced in turn,cyclically, and are translated, each through its respective signalchannel, to a sequentially operating signal gating switch S3 whichserves to connect each signal channel in turn to the input of thementioned amplifier, with a short interval of time elapsing betweendisconnection of one channel and connection of the next. Switch Sr isoperated synchronoously with commutator Cc by suitable mechanicalinterconnections, such as a shaft indicated by a dotted line, wherebythe output ends of the four signal channels are closed and opened at theproper times for timed acceptance of their signals in cyclicallyrepeated sequence by the amplifier. The latter, indicated at 25 on theblock diagram, amplifies the signals fed. thereto by switch Ss; andsince the signals have, by means 15, 16, etc., been reduced toindividual levels of comparable intensity well within the dynamic rangeof amplifier 25, are not significantly distorted. At the output of theamplifier the signals still are alternating in charcater; and forreasons hereinafter more fully explained, are converted into D.C. pulsesignals by a synchronous rectifier unit 26 which is operated at afrequency the same as that of the A.C. signals. The rectifier unit maybe operated by coil means as indicated, with power derived fromtransformer Tit through which the electrode current flows.

The signals appearing at the output leads of the synchronous rectifierare in the nature of DC. pulses, with a small A.C. componentsuperimposed thereon. These signals, four in each cycle of operation ofswitch Sr in the exemplary form of apparatus, are transmitted to thesurface apparatus through contacts of a relay R3 3 which serve toconnect the rectifier output to conductors Z and 3 of the cable asindicated. At the surface the 11C. pulse information signalsrepresenting the field potential intensities at the pick-up electrodesare translated through the slip rings and brushes at Sr at the winchdrum, to leads 2' and 3'. From the latter the information signals arepassed through normally closed contacts of a relay Ry2 and translatedthrough a signal converter means comprising a chopper 3d, a transformerT12, and a synchronous rectifier 31. The chopper converts each DC.signal pulse and the superimposed AC. components into short discretepulses which are transformed by T12 into A.C. waves of variousfrequencies. The basic AiC. wave burst appearing at the output of T12and representing only the DC. signal pulse component of the input intothe chopper 39, is re-converted into a reconstituted DC. pulse byrectifier 31, which acts to chop all other AG. components into burstsand discontinuous pulses of Very short duration. All but thereconstituted DC. signal pulses are then removed by a low-pass filter 32which translates on for switching and utilization only the DC.information signals. The latter signals are fed into a signaldistribution lead 33 from which each signal is selectively admitted,through a respective one of calibradon-attenuation nets 41, 42, 43, 44,and a respective one of gating relay units 51, 52, 53, 54 into anindividual one of signal pulse extender and amplier units 61, 62, 63,64. Each extender and amplifier unit includes means for effectivelyextending the duration of each admitted D.C. pulse for the period of acomplete 4-signal cycle so that from each amplifier unit there may besupplied to the respective galvonometer-recorder unit G1, G2, G3, or G4,a signal that changes significantly or is reset in value only once foreach signaling cycle.

Since switching of the signal pulses into the respectivegalvanometer-recorder units at the surface location requires that theswitching means comprised in the gating relay units be operated insynchronism with switch unit Sr of the subsurface apparatus, means areprovided for such synchronous operation. These means comprise acontrol-pulse generating means in the subsurface apparatus, means forphantom-circuit transmission of the control pulses to the surfaceapparatus, and means at the surface for receiving and there using thecontrol pulses to cause successive cyclically repeated operations of thegating relay units, whereby the first signal pulse of each cycle orgroup of such pulses is routed into galvanometerrecorder unit G1, thesecond into G2, etc. The control pulse producing means comprises a powersupply unit 35 supplied from transformer T11 in the subsurfaceapparatus, unit 35 supplying plus and minus D.C. potentials to a controlpulse generating rotary switch means 36 which is operated by motor unit7 synchronously with sequence switch Sr. The connections and operationof units 35 and 36 is such that there is supplied to a phantom circuitcomprising cable conductors 2 and 3 as one lead, and ground (cablesheath) as the other lead, repetitive series or groups of DC. controlpulses. In the illustrated system there are four DC. pulses in eachgroup, the first three being of plus polarity and the fourth of minuspolarity, as viewed from the cable conductors 2 and 3. The DC. controlpulses are fed in turn in cyclically repetitive manner to conductors 2-3through a lead 37 and a choke Chi connected to a tap on a resistor R101connected across leads 2" and 3", as indicated. Leads 2" and 3" areconnected to respective cable conductors 2, 3 through contacts of relayRy3. This phantom transmission scheme permits transmission of the syncor control pulses without interfering with the information signal pulsesconcurently transmitted on conductors 2 and 3.

At the surface apparatus the DC. control pulses appearing betweenconductors 2-3 and ground are recovered at the end of a terminating net45 connected across branches of leads 2' and 3 as shown. The series ofcontrol pulses thus recovered are fed through a filter unit 46 andtranslated to and reformed in a circuit unit 47, amplified by a specialamplifier unit 48, and fed to a control circuit unit 49 which producesnew DC. control pulses which are individually routed over respectivecontrol pulse leads to respective gating relay units as indicated. Thecontrol pulses there cause timed operation of the relay devices to passthe incoming DC. signal pulses arriving on lead 33 to their respectiverecorder units.

For a satisfactory log of NP. it is necessary to detect and indicate therelatively slowly changing variations of that potential; and since thesevariations fall in the frequency range from 0 to about 2 c.p.s.depending upon speed of logging, the natural potential signal may beconveyed to the surface on the first cable conductor 1., which is alsoemployed to convey alternating current from the surface to the loggingtool. Sheath and/or earth ground is the return path in both cases.Relatively simple filter means are employed at the surface apparatus toseparate the NP. signal from the higher frequency logging current; andsimilarly at the logging tool, filter means exclude the downgoinglogging current from the NP. electrode system, and the NF. signal fromentry into the power and current-electrode system. The NP. signalutilizing means, including filter and amplifier units and a recorder,are collectively indicated at NP. in the upper left in FIG. 1.

Other means and modes comprising portions of the invention notnecessarily indicated in FIG. 1 will be ex plained and describedhereinafter in connection with a detailed disclosure of the illustratedspecific exemplary and preferred embodiment of apparatus according tothe invention.

Referring now to FlGS. 2a and 2b in particular, the leads leading toconductors l, 2 and 3 and to the sheath S, of the cable Ca, aredesignated 1, 2', 3' and S, respectively. The respective conductors andcable elements are interconnected by conventional slip ring and brushmeans on the winch, these interconnection means being diagrammaticallyindicated at block Sr at the lower left in FIG. 2a. Alternating current,preferably but not necessarily of 400 cps. frequency, is supplied toconductors 1' and S through a synchronous converter drive coil Svd inconductor 1 and a direct current blocking capacitor C1 interposed inconductor S. As a consequency, coil Svd is normally energized during alogging traverse, and operates the reeds or movable contacts SvSl andSvS2 (FIG. 2b) of a synchronous converter unit, as indicated by thedashed control line leading from coil Svd. The purpose of operating asynchronous converter in positive synchronisrn with the alternatingcurrent supplied to conductors 1 and S will hereinafter be made morefully apparent.

The Constant-Current Generator (Unit 5) An electronic oscillator isemployed to generate an alternating current wave of the frequency (inthis example, 400 cps.) desired for the constant current supplied toconductors 1'-S. This oscillator (see upper left of PEG. 2a) is of theresistance-capacitance phase-shift type, comprising essentiallyresistors R1, R2, R33 and R4, capacitors C3, C4, C5 and C6, and electrontube V1 which has a plate load resistor R5. The oscillator output is onthe cathode side of V1 so that the frequency of the oscillator is notvaried by load fluctuations as it would if taken conventionally off fromR5, the latter being in the frequency determining portion of theoscillator circuit. The cathode circuit of V1 includes capacitor C7,resistor R6 and the primary TIP of a transformer T1 whose secondary T18is center-tapped, all as indicated, R6, C7 and TIP are of electricalvalues so chosen as to resonate at the selected frequency, 400 c.p.s'.The gain of the oscillator is adjusted by variation of a variableresistor R7 in the cathode circuit of V1; and the gain is set at a valuejust above that at which oscillation commences. A good sine wave form issecured from the oscillator, at T1. The output of T1 is applied througha unique four-terminal control device comprising a balancedvariable-conductance bridge network or regulator including the primaryT2? of a transformer T2, to the input of an amplifier tube V2. Theregulator device or circuit includes first and second semiconductordiodes D1, D2 which operate as variable-conduction units rather than asrectifiers, two resistors R8 and R9, and a capacitor C8 which with theprimary T21 is resonated at the selected frequency (400 c.p.s.). Theoperation of the regulator device in providing either constant currentor constant voltage output from the generator will hereinafter be morefully described. The output of the oscillator as evidenced attransformer T2 is applied to the grid circuit of amplifier tube V2, andthe output of the latter tube, developed across resistor R10, is appliedthrough capacitor C9 to the input of a two-stage network comprising anamplifying tube V3 and a cathodyne phase inverter not compris ing tubeV4. The output of tube V4 is applied through capacitors C11 and C11 to alow-power driver stage comprising electron tubes V5 and V6, and theoutput of that stage is applied through a transformer T3 to a pair ofpower output stage tubes V7 and V8 which provide power to the primary ofa transformer T4. In the preferred form of apparatus the lattertransformer supplies the constant intensity alternating current to theaforementioned conductors 1'-S through means elsewhere herein described.

As indicated in FIG. 2a, the output of tube V2 is applied to the gridcircuit of amplifier V3 across resistor R11, and the output of V3,developed across resistor R12 is coupled to tube V4 through capacitor Cand resistors R13 and R14. The split output of V 4, developed acrossresistors R14-R15 provides oppositely phased inputs for driver tubes V5and V6, whose grids are excited through capacitors C11 and C11 and arebiased through resistors R17 and R18, respectively. The cathodes of V5and V6 are connected to ground by way of resistor R19 and capacitor C12as indicated. A feed-back is provided from the secondary of transformerT3 to the cathode of V3, through resistor RM. The cathode of V3 isconnected to ground through resistor R21. Plate potential is supplied totubes V1, V2, V3, V4, V5 and V6 by way of a lead 1&1 connected by way ofan adjusting net (hereinafter described) to the output of a platevoltage supply means; and anode potential for tubes V? and V55 issupplied as indicated from the same supply means. Lead 1511 is providedwith an A.C. by-pass capacitor C13 connected between that lead andground.

The 400 c.p.s. generator output transformer T4 has one secondaryterminal connected to lead 1' through a current meter M and thesynchronous converter drive coil Svd, and its other secondary terminalconnected to capacitor C1 in lead S through a variable resistor R22which is used to provide a series sample of the current output of T4 forregulation purposes. As will be explained hereinafter, a shunt sample isused in those cases wherein output voltage intensity is to be maintainedconstant. The series control potential is derived across thecurrent-conducting portion of R22 and energizes the primary of atransformer T5 through leads 8, 9. The stepped-up output from thesecondary T58 is applied to a voltage doubler-rectifier and filter netfor derivation of a DC. regulator voltage for application to the:previously mentioned regulator device or variable conductance bridgenetwork. The doubleuectiiier and filter net comprises diodes D3 and D4and filter elements C14, C14, R23 and R24, with R23 providing overloadprotection for the di odes. The DC. regulator potential, derived fromthe junction of R23 and C14, is applied to the bridge circuit at thejunction of resistors R8 and R9 (which preferably are of equal value)through a lead 162, the thus-applied potential being applied inopposition to a selected D.C. reference potential which is applied tothe mid-point of T13. The value of the reference potential thus appliedin opposition to the regulator potential is selected by variation of apotentiometer resistor R25 which is connected between ground and apotential divider connected to a voltage regulator Vld. Lead 1th. issupplied at, for example, 300 volts DC. potential, whereby, by means ofpotential-dropping resistors R26 and R26, with regulating gas-filledtubes V9 and V10, there is supplied to the high potential end of R25 aDC. potential of, for example, 10 volts. The regulator circuitryoperates in the following manner to maintain constant current outputinto conductors 1S': with an amrneter M inserted in the output of T4,R25 is adjusted until the meter indicates the desired value of electrodecurrent. This current flow provides a sample voltage drop through theactive portion of seriesconnected resistor R22 exactly sufiicient, whendoubled and rectified and filtered by D3, D4, C14, C14, R23 and R24, andapplied through lead 102 and R8 and R9, to balance the potential pickedup on R25 and applied to the midpoint of secondary T18. In this balancedcondition the conductivity of D1 and D2 (which as hereinbefore noted donot operate as rectifiers) is such as to permit transfer of exactly theamount of energy or power from T18 to T21 necessary to cause the powersupply unit controlled by V2 to provide the selected output currentthrough the secondary of T4. If the latter output tends to commence todecrease, due to an increase in formation resistivity adjacent thelogging tool or due to increase in cable conductor resistance, etc., thesample potential picked off from resistor R22 similarly tends to drop,causing an unbalance of the DC. potentials applied to D1 and D2 and inthe direction to increase their conductivity, whereby transfer of energyfrom T1 to T2 increases and the output of T2 is increased to bring up,through the action of V2, V3, V4, etc., the output potential of thepower supply at T sufiiciently to overcome the tendency of the outputcurrent to decrease. Similar- 1y, if the output current at T4 tends toincrease, due to a decrease in current path resistance in the cable and/or earth formation, the increasing potential picked up at R22 causes anopposite unbalance of potentials applied to D1 and D2, lessening theoutput at T2 and, through V2, etc., decreasing the output potential ofthe power supply at T4.

The control or current regulating system whose operation is described inthe preceding paragraph is automatic, and, quite unlike carbon pile andother current regulators, is substantially instantaneous in its actionand independent of phase characteristics of the load and not subject tothe loop oscillations of conventional A.C. servos. As is evident, orwill hereinafter become evident, such substantially instantaneouscurrent-value correction is requisite to the extremely arcuate loggingprovided by applicants exemplary system. The control device orregulator, employing the diodes D1, D2 as variable conduction devices(hence silicon junction type diodes, rather than germanium diodes, areused) in a balanced network, not alone provides instantaneous control,but eliminates the distortion incident to use of a single diode controlcircuit. In explaining in further detail the functional operation ofdiodes D1, D2, reference is made to FIGS. 4 and 4a. In FIG. 4- isdepicted a typical voltage-current or conduction characteristic curvefor a diode, plotted on Cartesian coordinates representing voltage (E)and current (I) as indicated; and in FIG. 4a is depicted a considerablymagnified central portion of FIG. 4,illustrating the portion of thecharacteristic curve over which operation of diodes D1, D2 is limited inoperation of the current generator regulating circuit of the invention.It will be noted that the characteristic curve is that of a diode havingsubstantially no contact potential, i.e., such as that of a germaniumdiode. The curve, in the region very near the origin of the coordinateaxes, approaches closely a straight line, as is indicated in themagnified part illustrated in FIG. 4a showing the part of the curveextending from E=.3 v. to E=+.3 v. in full line and parts of theextension of the limbs of the curve above and below those values indotted lines. The electrical values of the regulating circuit and thepotentials supplied thereto are so chosen as to restrict operation ofthe diodes D1, D2 to a portion of their characteristics well within thesubstantially linear section above and below the zero of the coordinateaxes. Since the midpoints of secondary T18 and primary T21 (PEG. 2a) arein effect grounded for the 406 c.p.s. Wave, and since that Wave is but avery small part of the current passed through diodes D1, D2, the A.C.wave cannot seize control of the control circuit. When the D.C. samplepotential (derived from R22) is negaive with respect to the referencepoential derived at R25, the diodes operate on that part of thecharacterisic extending from and to the right of the coordinate axes inFIG. 4a, and conversely for the lower limb of the characteristic;whereby in the former case the diodes are more conductive and moresignal is supplied to V2 to increase the generator output, and in theother case the diodes are less conductive and supply less signal to V2,to decrease the output. It will be understood, of course, that it is theoutput voltage across the secondary of T4 that is varied, so the currenttherethrough is maintained constant within a very narrow range ofvalues. It should be noted that if it is desired to provide aconstant-voltage output from the generator, the sample voltage should betaken from a resistor shunted across the generator output, rather thanfrom a series-connected resistor as shown.

Anode potential is supplied to the previously mentioned lead 101 throughan adjustable filter network comprising capacitors C13, C15, variableresistor R27, and a choke C112, from a supply unit comprising rectifiertubes V11, V12. The rectifier cathodes are energized from the secondaryT651 of a transformer T6, and the anodes are energized by acenter-tapped secondary T78 of a transformer T7. The primaries of T6 andT7 are connected to any suitable A.C. supply line, such as a 115 voltAC. line as indicated. Additional secondary coils of transformer T6 areprovided for supply of current for electron tube filaments and heatersof the current generating unit, as indicated.

12 The Subsurface Apparatus Referring now to the composite formed byFIGS. 3a and 3b, cable conductors 1, 2 and 3 and sheath conductor 8 arediagrammatically indicated at the left of FIG. 3a. The 400 c.p.s.alternating current traversing conductors 1 and S may be traced fromconductor 1 through D.C. blocking capacitor C2, through the primaries oftransformers T10 and T11 in series, on through. driving motor M0 of unit7 (FIG. 3b) and into the rotary member or brush of a current commutatingswitch Cc. From the rotary brush the current passes through,successively, stationary contacts a, b, c, and d and to respective leads111, 112, 113, and 114. The latter leads are connected to respectivemovable contacts e, f, g, and h of a multideck rotary switch device Rslhaving an actuating or stepping coil RslC and shaft means (indicated bydotted line) for operating the switches of the several decks. In thenormal or operating position of Rsl as shown, the current pulsescommutated into leads 111, 112, 113 and 114 pass into respective currentelectrodes Be, Be, E1,

and Eg which are diagrammatically depicted at the lower right in FIG.3b. As hereinafter more fully explained, the commutation of theelectrode current is so accomplished that there is no interruption ofcurrent flow through the cable conductors 1 and S. This is due to aslight overlap of the rotary brush of the commutator Cc with adjacentstationary contacts a, b, c, and d as the rotary brush passes from oneto another of the stationary contacts. The current continues (nowdiverted cyclically into four different paths), through the earthadjacent the current electrodes, Ec, Be, 15 and Eg, and to a relativelyremote ground terminal formed by an exposed, uninsulated portion of thecable sheath conductor S disposed within the liquid-filled borehole.

The electrode system provided on the subsurface tool which is traversedalong the borehole may be as desired or required for the types of logsrequired. In the apparatus herein disclosed by way of preferred exampleand which is adapted to secure information for four resistivity logs andan N.P. log, seven electrodes are situated specific distances De fromthe lower end of the subsurface tool according to the following table:

Electrode: De, 1n

Ea 0 Eb 8 Ec 16 Ed 32 Be 64 E 136 Eg 240 It is understood that theelectrodes are insulated from the body of the tool in accord withconventional logging tool construction practice, and that electrode Eais as close as practicable to the bullhead end of the tool body (hereinconsidered to be at zero distance from the tool end). The exteriorsurface of the tool body is insulated from borehole fluid, as is a lowerend portion of the cable sheath, Si (for example, the lowermost feet ofthe cable is enclosed in an insulative jacket), whereby groun for theelectrode cur-rents is relatively remote from the logging toolelectrodes. Electrodes Eb is in this example employed for detection ofNP. In the exemplary form of apparatus depicted, the cable sheath S, asa ground, is used as a return electrode for current for the shortnormal, long normal, short later-a and long lateral resistivity logs;and the potential pick-up for the lateral resistivity log informationsis between electrodes Ea and Ed with the potential pick-up for thenormal resistivity log informations taken between electrodes Ea andsheath S. For convenience in reference, the current electrodes (C111 andCuZ) and the potential electrodes (Pi and F2 for the four resistivitylogs may be tabulated as follows:

The potentials forming the information required for production of theresistivity logs, in the form of 400 c.p.s. waves detected at thepick-up electrode pairs, are detected at and derived from the electrodesas indicated in the preceding table, and in the order there listed; andthe potentials are translated through respective decks of rotary switchdevice Rsll and registered across separate respective input circuitsthrough individual attenuation devices. The input circuits compriseprimaries of respective isolation transformers T21, T22, T23, and T24.The secondary windings of the latter transformers are connected, insequential order by a comnrutating switch Ss, to a common amplifier forsignal amplification. The amplification is preparatory to conversion ofA.C. signals to DC. signal pulses which are in turn impressed uponconductors 2 and 3 of the cable. During a normal logging traverse themovable or rotary contacts of all banks or decks of switch Rsl are incontact with respective stationary contacts herein illustrated as theupper, or X contacts of series of three such sets of contacts, X, Y, andZ. Thus the X-position of switch Rsl is the normal operating position.Means whereby the switch may be rotated at will under control of theoperator will hereinafter be disclosed; it being sufficient at thispoint to note that the swich may among other things be employed to bringinto action certain indicating means whereby positional status ofcomponents or" the subsurface apparatus may at will be determined duringa logging traverse. The attenuation means mentioned comprises four setsof resistors, R01, R02, R03, and R04, one set for each of theresistivity information signal channels, in the order previouslymentioned; and each set comprising resistors wih sub-designations at, b,c, d, e, and f as indicated in set R03.

It will be noted that the potential in the first resistivity loginformation channel (short normal), picked up between electrodes Ba andS while the current electrodes Be and S are conducting, is concurrentlyapplied through attenuation means Rel to transformer T21 and through R02to transformer T22; but since T22 has its secondary circuit open atcommutating switch Sr (b) at that time, the signal will be passed intothe movable brush of Ss through only T21, it being noted that thesignals progress from right to left in FIGS. 3b and 3a. Similarly, thepotential in the second resistivity log information channel (longnormal), picked up between the same electrodes Ea and S, is concurrentlyapplied to both of transformers T21 and T22; but at that time thesecondary circuit of T21 is open at Ss (a) and the signal is passedthrough only T22. Similar considerations apply with respect to the thirdand fourth resistivity information channels, the signals therefor beingpicked up between Ea and Ed and passing respectively through T23, andT24 into the rotary brush of Ss. As indicated, each of sets Roll, R02,R03, and Rod comprises a plurality of resistors (for example, sixresistors). At any logging traverse of a given extent of borehole, onlyone of the resistors of a respective set is connected in series with theprimary circuit of a corresponding transformer. The purpose of theselected resistor in each bank is to attenuate the AC. signal passingtherethrough to a predetermined extent whereby at the output sides oftransformers T21, T22, etc., the signals in the four separate channelswill be attenuated to amplitudes Within a signal-level range that canreadily be accommodated by a single, common, amplifier to which all ofthe signals are fed for amplification. it is evident that the intensityof the potential picked up for the short normal log (between Ea and Swhen current is traversing electrodes Ec and S) is much greater thanthat picked up for the long normal log, and many, many times greaterthan that picked up for the lateral logs. Since economy ofpower-consuming electronic apparatus in the logging tool dictates use ofa common signal amplifier for all the signals, the inputs to theamplifier from the separate signal channels are purposely attenuated tointensity levels well within a range accommodated by a single amplifier.The several resistors comprised in a given attenuation means Rel, etc,are of values so selected that the log produced from a given electrodeconfiguration may be of any prescribed or selected sensitivity within agroup of sensitivities determined by the resistor values. Thus forresistivity logs of li'J-ohmmeter sensitivity, the lowermost resistors(a) of the four sets, shown connected to the transformer inputs, areused. For 20 ohrnmeter logs or curves, the next resistor (b) in each ofthe groups is switched into the input, etc. For sensitivitiesintermediate the values provided for by the several resistors in eachattenuation sets R01, R02, etc., a circuit means is provided in thesurface apparatus. For example, if resistors Rcla, Roll), etc., of R01provide respectively for sensitivities of 10, 20, 40, ohmmeters, etc., alog may be produced at 15 ohmmeters sensitivity by utilization of thementioned surface circuit means. The latter will hereinafter be morefully described and explained. Selection of a resistor from each of setsRel, R02, etc, is by means hereinafter described.

The 400 cps. wave signals representing the information for theresistivity logs and translated along first, second, third, and fourthsignal channels respectively comprising transformers TZFl, T22, T23, andT24, are in succession picked up by the rotating brush or contact ofswitch Sr from contacts Ssa, Ssb, etc, and are introduced or applied tothe common signal amplifier by way of the primary of an inputtransformer TM to which the rotary brush is connected. An interferenceor are suppression capacitor C21 is provided between the moving contactof switch Sr and a floating return lead 116 to which intercontactshorting bars of switch Ss are connected. By the provision of theshorting bars in switch Ss, and the connections indicated, substantiallynot ing but bursts of 400 c.p.s. signal are impressed upon the amplifierthrough input transformer TM.

The subsurface signal amplifier consists essentially of a two-stagesignal amplifier comprising electron tubes V129 and V21. The signal isimpressed upon the signal grid of V23 and the amplified signal coupledthrough capacitor C22 to the grid of V21. The output of V21 is coupledto rectifying means in unit 26 for converting the bursts of 400 c.p.s.signal into direct current pulses. The coupling is by way of atransformer T15 into whose primary the output of V21 is passed. Thesecondary of transformer T15 is center-tapped and is connected acrossthe fixed contacts of a synchronous rectifier means Sr having a movablecenter contact or blade Srb driven by a coil Src which is energized by400 c.p.s. power derived from the secondary of the aforementionedtransformer Tilt. The synchronous rectifier serves to rectify the 480cps. signal waves into direct current pulses which are applied throughleads 1231, 122, and appropriate sets of contacts Ryfirz, Ryfib of arelay, Ry 3, to conductors 2 and 3 of the cable. The direct currentpulses, whose respective amplitudes mathematically represent (accordingto the ratios of the active resistors in Rel, RC2, etc.) the intensitiesof the signals picked up at the potential electrodes, are transmitted asthe information-representing signals from which four resistivity logsare to be derived or produced by the surface apparatus. The directcurrent signal pulses are transmitted in groups of four, one group foreach rotation of switches Cc and S5, with each signal group containing asignal for each of the resistivity information channels. Switches Sc andSs are synchronously operated with a third rotary switch, 36, by acommon switch shaft indicated by dotted line Ms. The switch shaft isdriven at a substantially constant speed of rotation by theaforementioned motor M0, which may include as part of unit 7 aspeed-reducing gear box. The mentioned third rotary switch, 36, isemployed for producing what may be termed control pulse or sync signalsfor synchronizing certain of the surface-apparatus operations withcertain subsurface-apparatus operations, as will presently be describedand explained. Switch Ss, which samples the four resistivityinformations or signals, each in turn at a rate of one sample perchannel per revolution of shaft Ms, is provided with short-circuitingbars mechanically situated between the signal-conducting con tacts Ssa,Ssb, etc., whereby the four resistivity signals translated intotransformer T14 are discrete, time-spaced, 400 cps. signals, eachcompletely free of any interference from the others. The character ofthe four signals entering T14 is indicated in FIG. which shows exemplarysignals Sil, S12, Si3, and S14.

The resistivity signals, after amplification by V29 and V21 andsynchronously rectified by Sr, are in the form of discrete time-spaceddirect current pulses, such as Si1', Si2, S13, and SM, indicated in thelower part of FIG. 5. The synchronous rectifier, is operated by powerderived from the same current that is emanated from the currentelectrodes, hence is operated synchronously with the 400 c.p.s. signalspicked up at the potential electrodes. As previously mentioned, thedriving power for the synchronous converter coil is derived throughtransformer T; and to attain maximum utilization of available Signal,the phase relationship of the driving power may be suitably adjusted bya phase-shifting network 33 comprising capacitor 625 and a variableresistor R35. Thus the vibrating con-tact Srb of the converter may becaused to open and close with the opposed fixed contacts at times suchas to recover a maximum of signal energy with a minimum of current breakat the contacts. A small component of 800 c.p.s. ripple is, of course,superimposed upon the DC. pulses applied to leads 121, 122 by theconverter; but this is of no material consequence and is readily removedfrom the signals by filter means in the surface apparatus, as willpresently be described and explained. The DC. signal pulses aretransmitted through assigned contacts of relay Ry3, which as indicatedis energized to hold the DC. signal circuit closed at all timesalternating current passes to the current electrodes through the primaryof transformer T11. As will hereinafter be explained in connection withthe calibration means and procedure of the invention, relay Ry?) is alsoemployed to connect conductors 2 and 3 to switch-controlling andindicating circuits when the alternating current supply to conductor 1is opened or terminated.

As is evident from an examination of PEG. 3a, transformer T11 isemployed not alone for energizing relay Ry3 through a rectifier meansRep comprising a rectifier tube V22, but also for supplying power forthe aforedescribed signal amplifier and for the sync or control pulsesproduced by the action of switch 36. Filament power is derived from anauxiliary secondary T11s2 of T11; and direct-current power is providedthrough filter means indicated at Fis, to the amplifier anode circuits(via lead 124), and to switch 36. The power supply circuitry includesresistors R36, R37 bridged in series across the rectifier output, andthus a neutral lead 126 connected to their midpoint may be provided,with respect to which leads 127-124- are positive and lead 116 isnegative. Neutral lead 126 is connected to the cable sheath through alead 126' and to the rotary contact of switch 36 through a lead 126".The first, second, and third fixed contacts of switch 36 are connectedas indicated through lead 127' to positive supply lead 127; and thefourth fixed contact of switch 36 is connected by a lead 116 to negativesupply lead 116. Thus as the movable contact of 36 rotates,

between electrode Eb and ground appears between conlead 126 is for threeseparated periods made positive and then for the fourth period madenegative, with respect to the cable sheath. Lead 126" is connectedthrough choke coil C121 to an adjustable tap on resistor R101 bridgingleads 121 and 122. Thus there is applied to cable conductors 2 and 3(considered as a single conductor) and the cable sheath, repetitiveseries of DC. sync or control pulses, each series comprised of first,second and third pulses of positive polarity and a fourth pulse ofnegative polarity. A complete series of these control pulses isdiagrammatically depicted below the horizontal reach of lead 126" inFIG. 3a. The pulses are suitably separated in time, as indicated in thediagram; and are transmitted to the surface appartus over cableconductors 2 and 3 as a phantom lead and the cable sheath. The means andmode for utilizing the control pulses are hereinafter explained.

A natural potential (N.P.) signal is picked up between electrode Eb anda ground electrode provided at the surface of the earth. This signal, ofslowly varying D.C. character with significant variations in the 0 to 2c.p.s. range, is passed through chokes C114, C125 (FIG. 3b), and a lead129, to cable conductor 1. The N.P. signal is prevented from enteringthe A.C. power lead in the subsurface apparatus by D.C. blockingcapacitor C2.

The DC. information signal pulses (with a small superimposed A.C. wavecomprising harmonics of the 400 c.p.s. signal wave) and the DC. controlpulses as well, are somewhat distorted in the court of transmission tothe surface apparatus due to the inherent characteristics of the cable.However, as will presently be made evident, this distortion is notdetrimental in the case of either the information signals or the controlpulses, be cause both the distorted leading and trailing portions of theinformation pulses are eliminated, and the control pulses are employedonly in the creation of relay-operating pulses of distinctly differentcharacter.

Surface Apparatus, Signal Reception and Utilization Referring now toFIG. 2a, the NE. signal developed ductor 1 (lead 1') and theaforementioned surface ground electrode inserted in the earth anddesignated G(Sur). The NP. signal current flows through conductor 1,lead 1, coil Svd, meter M, secondary of T4, part of resistor R22, and alead 13% to normally closed contacts Ry4a of a normally relaxed relayRy4. From the latter the current flows through a resistor R46 and chokeC116 of filter Fil, through a resistive net comprising adjustableresistance R31, to N1. recording galvanometer NPG; and the currentreturns from NPG to the surface ground electrode G(Sur) by way of thelowermost of relay contacts Ry4a. Thus in normal logging operations anN1. curve or log is obtained, it being understood that the record mediumis moved in proportion to traverse of the tool Lt through or along theborehole in a well known manner by known means.

The upcoming resistivity information signal pulses arriving on cableconductors 2 and 3 are translated through the slip ring and brushstructure of the cable winch and onto leads 2 3 (FIG. 2a). These DC.pulses are passed through normally closed relay contacts Ry4b of relay R14, and leads 2"-3", to the input of the synchronous chopper-rectifierSv (FIG. 2b), driven by the previously mentioned coil Svd. The inputcontacts of this device, operating at the frequency of the A.C. throughcoil Svd, chop each of the incoming DC. signal pulses into many DC.pulses of briefer duration, and the latter are applied oppositely inalternation at the chopping rate to the center-tapped primary of atransformer T12. Thus the principal ouput at the secondary of T12 isbursts of 400 c.p.s. alternating current of approximately square waveform; one burst for, and of intensity comparable to, a respective D.C.input pulse. The 800 cps. wave of small magnitude that was transmittedwith and superimposed on the DC. signal pulses in the subsurfaceapparatus, is also chopped by the chopper section SvSZ of Sv and appearsat the secondary of T12 as an assemblage of 800 c.p.s. waves andharmonics thereof. The output of T12 is rectified by a second set ofcontacts $1182 of converter Sv, the two fixed contacts thereof beingconnected across the secondary of the transformer and the output beingtaken off the vibrating contact and a midpoint tap on the secondarywinding of T12 as indicated. Since the DC. chopping at SvSll and therectification at 81/82 are elfectecl synchronously by concurrentoperation of the two movable contacts by coil Svd, as indicated by thedotted lines interconnecting the respective named elements, the outputat SvSZ is a combination of a series of DC. pulse signals and bursts ofextraneous AC. Waves of 800 c.p.s. and higher frequencies superimposedon the DC. signal pulses. The desired signal output is of much greaterintensity than the extraneous unwanted A.C. component, and the latter ifreadily separated from the former by appropriate conventional filtermeans such as that shown at 32 and including choke Ch? and capacitorsC32, C33, and C34. It may be here noted that the synchronous choppingand l'C-IBCtlilCZtilOIl and filtering of the upcoming DC. pulse signalsprovides a novel and highly efficient mode of eliminating all A'.C.components from the signal, since the chopping cuts all of the incomingwaves and signals into bits, converting each wanted DC. signal into whatmay be termed a modified 40-0 c.p.s. square wave AC. signal, andconverting all the undesired A.C. input components into short bursts ofAC. wave of 400 c.p.s., 8G0 c.p.s., and higher frequencies. The desiredsignal component appearing at the output of T12 in the form of bursts of400 c.p.s. square wave signal, is rectified by SvSZ into discrete DC.pulse signals. Since the desired part of the output of 81/82 is now inthe form of discrete D.C. pulses and all of the undesired part of theoutput is A.C., the latter is readily eliminated by the describedfilter. The original isolated ungrounded DC. signals are at this stageconverted and referred to ground for easy amplification by meanshereinafter described. The filtered output signal comprises onereconstituted DC. pulse for each resistivity information channel in eachcommutaton or signaling cycle; that is, the signal appearing on lead 33(FIG. 252) at the output of the filter unit 32 is in the form ofrepetitive groups of four time-separated D.C. pulses per group. Thefirst, second, third and fourth pulses of each group are by meanspresently described switched or routed into respective individualresistivity signal channels for utilization in producing theaforementioned short normal, long normal, short lateral and long lateralresistivity curves or logs.

Surface ApparatusContrl Pulse Utilization Referring again to FIG. 2a,and recalling that so-calied sync or control pulses produced by actionof switch as in the subsurface apparatus were applied between the cablesheath S and conductors 2 and 3 as a phantom pair, the DC. pulses thustransmitted appear at the surface apparatus between conductors 2 and 3(and 2. and 3') as one lead, and the sheath ground as the return lead.These DC. control pulses are extracted at the surface apparatus byconnecting one lead to ground (sheath) and another lead to a terminationnetwork 45 interconnecting leads 2' and 3. The termination networkcomprises resistors 5h, Sit connected in series between leads 2' and 3'to provide a mid-point junction to which a control pulse lead 15%) isconnected. The control pulse signals in repetitive groups of threepositive pulses and one negative pulse appear across resistor 52connected between lead 15%) and ground, and a portion of the thusmanifested control signals is applied, through appropriate AC. rejectionfilter means PH 5, to novel circuitry which reshapes the pulses toprovide precisely timed pulses of special electrical configurations, thelatter pulses being in turn amplified and used in another novel circuitto control operation of respective signal gating relays interposed inthe four resistivity signal utilization circuits. The latter relays areso operated as to perform the signal routing function mentioned in thepreceding paragraph. Filter Fl! 5' comprises a twin-T network to remove400 c.p.s. cross-talk, and other filter elements to remove extraneousA.C. potentials. The twin-T net comprises capacitors Chi), C61, and C63and resistors R53, R54, and R55; and the remaining filter elementscomprise C64 and C65 and resistors R56 and R57.

The control signals, in repetitive groups each comprising three pulsesfollowed by one pulse, as they appear at the outlet of filter Fil 5, areof character indicated by the wave form depicted adjacent to that filterin FIG. 2a. The distortion from the originally created square wave formis due to transmission through and from subsurface apparatus to theoutput side of filter F1! 5. By operation of means next to be describedand for purposes and reasons presently explained, the positivegoingpulses are separated from the negative-going pulses and are passed intoand through respective novel pulse reformation circuits wherein arecreated new corresponding pulses of extremely sharp wave front. Thepulses of both polarities appear between ground and the junction ofRti-Rd? at the output end of filter F1! 5. A rectifier Rail connectedbetween lead 152 and ground lead 153 shorts out or eliminates allnegative-going pulses from the input circuit of a positive-pulseamplifier tube V24 which is normally biased close to the cut-off point(near or at the non-conducting state). Thus only the positivegoingcontrol pulses are effective in causing or increasing conduction throughV24. A second control pulse amplifier tube, V25, has its input circuitconnected to lead 151; and this tube is normally operating at zero bias,i.e., is normally fully conducting. Thus the positive control pulsesappearing on lead 151 do not significantly change the conduction statusof V25; however, arrival of the negative-going (fourth) control pulse onlead 151 causes V 25 to cease conduction for a brief interval. Thenormal negative bias for V24 is supplied by means of resistors R58, R59connected between a anode voltage supply lead 155 and ground, with acathode conncction to V24 as indicated. V24 has an anode load resistorass connected to supply lead 155; and similarly, V25 is provided withanode potential through a load resistor R62 likewise connected to lead155.

In consequence of the circuitry and connections described in thepreceding paragraph, arrival of each 4- pulse group of control pulsesinitiates one of repetitive cycles of events and sequential controloperations. For convenience in describing those events and operationsand the apparatus involved, the control pulses will be numbered in theorder of their creation and arrival at the surface apparatus. Thuspulses numbers 1, 2, and 3 are of polarity and pulse number 4 isnegative.

When control pulse number 1 arrives at VT24 it causes conduction throughthat tube, and the resulting increasing voltage drop across use causes,at a certain voltage drop value, breakdown and conduction through a neontube Nel connected as indicated. The latter tube ignites or passes fromthe non-conducting state to a fully conducting state within an extremelyshort period of time, for example, within one microsecond. Referringalso to FIGS. 6 and 6a, the graph of FIG. 6 illustrates the wave form ofa pulse as applied to the input of V24, and FIG. 6a illustrates theconcurrent voltage across Nel. The circuit elements are chosen to be ofvalues such that 'as the voltage across R60 reaches a selected valuemore than 30 volts above the extinction voltage (Ext) of Nel, the latterconducts. The ignition voltage level is indicated as Ig on FIG. 6a, andthe extinction voltage at Ext. At the instant Nel conducts, the voltageacross Nel suffers a very rapid drop of, for example, about 30 volts toa conduction level indicated at Con, in a period of approximately onemicrosecond, thereby creating on lead 156 an extremely sharpnegative-going output pulse. The latter pulse, translated through acoupling capacitor C66, is applied to the input circuit of an amplifiertube V26 for amplification and phase inversion. At the output of V26,across anode load resistor R66, there is thus produced a sharp decreasein voltage drop (rise in potential) on lead 157. That is, a very sharplyrising pulse is produced on lead 157. Conduction through N21 terminateswhen the trailing end of the incoming pulse drops to the extinctionlevel Ext, as indicated in FIG. 6a. The justdescribed circuit operationsare repeated for each of the incoming control pulses, whereby thereappears on lead 157 a series of three time-spaced pulses of very steepwave front.

Arrival of a negative-going sync or control pulse from the subsurfaceapparatus initiates a somewhat similar sequence of circuit operations,but through a separate chain of elements, to produce a reformedsharp-front pulse for use with the three preceding pulses in operationshereinafter described. The incoming control pulse has no appreciableeffect on V24 since that tube is normally biased substantially tocut-off. The pulse is, however, applied via lead 151 to the input ofamplifier tube V25, which, as before stated, is normally conducting. Thenegative pulse briefly terminates or greatly decreases conductionthrough V25 and thus causes a rise in potential at the anode of V25.This change of potential follows closely the form of change previouslydescribed in connection with FIGS. 6 and 6a, and causes, after adetermined rise in potential, brealodown and conduction through a neontube Ne2. Conduction through Ne2, increasing to full value Within onemicrosecond, produces a positive-going pulse of extremely steep Wavefront across resistor R67, and this pulse is translated throughcapacitor C66 to the input of a triode V27. That triode is normallybiased to cut-off by a voltage derived through a rectifierresistor netcomprising resistors R68, R69 and a rectifier Re9 which is connected atK to one of the low voltage filament power sources of the surfaceapparatus. The terminal K may be located at transformer T20 in the upperportion of FIG. 2b. With V27 normally cut off, arrival of the sharppositive-going pulse created by conduction through R67 causes momentaryconduction through V27 and anode load resistor R72 (FIG. 2b). Thiscauses a negative-going pulse to appear on lead 160 at the lower end ofR72. This pulse is very sharp and is employed, following, and inconjunction with, the three pulses previously produced on lead 157, tocontrol a novel ring or relay control circuit employed to causeoperation of the four signal channel gating relays.

- The aforementioned ring or relay control circuit comprises fourtriodes V28, V29, V30 and V31 (FIG. 217), all operated on a commoncathode bias. The bias and operation of the control circuit is such thatonly one of the four triodes conducts at any time (except during veryrapid shift of conduction status from one triode to the next in thering); and further is such that the tubes conduct in turn, V28 inresponse to the aforedescribed negative pulse on lead 157, V29 inresponse to the first pulse, V30 in response to the second pulse and V31in response to the third pulse, assuming initial conduction through V28in response to a pulse. In effect, the negative pulse may be termed areset pulse for the reason that its arrival causes V28 to seize theconduction status from either of the other three triodes (V29, V30 orV31) that happens to be conducting. Thereafter the first pulse causesV29 to conduct, etc., in the order named. This will hereinafter be morefully explained in connection with a detailed exposition of thecomponents and operation of the relay control circuit.

As before noted, the reformed sharp negative pulse each group of foursuch pulses is applied to lead 160. That lead is connected directly tothe anode of V28, to the grid of V29 through an RC net comprisingresistor R81 and capacitor C69, to the grid of V30 through R85,

and to the grid of V31 through R89. The reformed positive pulses areapplied in timed succession to lead 161 from lead 157 through couplingcapacitor C68. Lead 161 is connected to the grid of V28 through R73, tothe grid of V29 through R78, to the grid of V30 through R83, anddirectly to the anode of V31. The anode of V28 is connected to B+voltage supply lead 155 through resistor R72; and the anodes of V29,V30, and V31 are connected to the same B+ lead 155 through,respectively, resistors R77, R87, and R88. The grids of V28, V29, V30,and V31 are connected to ground lead 153 by Way of respective gridresistors R76, R79, R84, and R90. The cathodes of the four triodesV28-V31 are collectively connected to ground lead 153 through anadjustable cathode bias resistor R86, and grounded for AC. potentials bycapacitor C72. Lead 160 serves as a control signal lead for V28, andlead 161 similarly serves as a control signal lead for V31. Similarcontrol signal leads 162 and 163 are provided for V29 and V30,respectively. As will presently be made evident, these control signalleads convey respective relay control signals produced by operation ofthe corresponding triodes, and serve also to convey respectivetriode-operation controlling voltages as Well.

The sequential operation of the ring or control circuit comprising thefour triodes is as follows, assuming that either one of the triodes isconducting, and a negative sync pulse is the next pulse to arrivethrough V27. If V28 is conducting, arrival of the pulse does not shiftconduction status to another of the four triodes, since that negativepulse is not applied to V23 but is applied to the grids ofnon-conducting tubes V29, V30, and V31 to insure continuednon-conduction thereof. However, if initially either V29, V30 or V31 isconducting (in which case V28 must be non-conducting), arrival of thenegative pulse on lead 160 causes the following actions: the pulse isapplied to the grid of V29 through paths including RR83 lead 161R78; isapplied to the grid of V30 through R85, and is applied to the grid ofV31 through R89. Conduction through whichever of V29, V30 or V31 isconducting is thereby decreased to some extent. Concurrently with thisdecrease in current through the conducting triode, a decreasing voltagedrop across the common cathode bias resistor R86 has the effect ofdecreasing the bias on the cathodes of all four triodes, and this inturn makes the grid of V28 more positive with respect to the cathode ofthat tube, thereby permitting conduction through V28 to commence.Conduction through the non-conducting triodes among V29, V30, and V31will not at this time commence, because of the negative pulse appearingon their grids While that is not the case with V28. As conductionthrough V28 commences and increases, restoration or normal cathode biason all four triodes commences and continues, with concurrent decreasingconduction through the previously conducting triode among V29, V30, andV31. The shift in conduction status from the previously conductingtriode to V28 is aided by the increase in current through V28, sincethat current produces a continuing and increasing voltage drop acrossR72, which negativegoing voltage is concurrently applied from lead 160to the grids of V29, V30, and V31 to further depress conduction throughthe conductive one of those triodes. Thus the shift of the conductivestatus to V23, once initiated by the incoming negative pulse, progresseswith increasing rapidity and is quickly accomplished. In fact, the shiftis so rapid as to be measured in terms of electron-transit time in thecontrol circuitry.

During the period V28 is not conducting, a charge builds up and ismaintained on C69 by current flow from ground through R79 and from B+lead 155 through R72 and lead 160. A similar charge builds up on C70 bycurrent flow from ground through R84 and from lead 155 through R77 andlead 162. Similarly, a charge builds up on C71 by current flow fromground through R and from lead through R87 and lead 163. Thus the triodegrids connected to the lower sides of C69, C70, and C71 are held atrespective negative b'as levels which may be individually lowered bypartial discharge of the respective capacitor. When conductive status isseized by V28 incident to arrival of the negative sync pulse, anodecurrent flow through R72 causes a drop in potential on lead res which ismaintained as long as Vs?) conducts. This action permits partialdischarge of capacitor C69 tlrough resistor R81, without, however,affecting C70 or C71; and this partial discharge of C69 and reduction ofnegative bias on the grid of V2@ sets the stage for, and insures,seizure by V22 of conductive status from V255 upon arrival of the firstsyn pulse on lead 161. The aforementioned voltage drop produced on leadincident to conduction through VZS is additionally and primarilyemployed to control operation of signal gating relay R 11 (upper rightportion of FIG. 2b) in the first resistivity signal channel, as willpresently be explained.

With V2 in the conductive state, arrival of the first sync pulse on lead151 initiates a shift of the conductive status from V225 and V29. Theincoming pulse is applied to the grid of V28 via R73, to the grid of V29via R78, to the grid of V3h via R83, and to the grid of via pathsincluding the path R73---R75lead l Z-RJ l; and of course the pulse mayarrive at the four grids by other paths which are obvious. Since all ofthe reformed pulses are of extremely steep wave front, there issubstantially no delay nor attenuation in their application to theeveral triode grid circuits. The noted pulse has no appreciable directeffect on V28, which is already conducting. Since by previous partialdischarge of C59 with concurrent maintenance of charge on C7 a and C71the negative bias on the grid of V29 has been lowered below that on V3and V31, arrival of the pulse on the grid of V29 raises the gridpotential at that point suificiently to initiate conduction through V29.Such conduction has two immediate effects both of which tend toreduction and extinction of conduction through V23 and concurrentlyinsure that neither of VEitl nor V31 will start conducting. The first ofthese effects is a lowering of the positive potential level of lead 162by the voltage drop through anode resistor R77, as conduction throughV29 commences, resulting in a decrease in the potential applied through317 5 to the grid of V28 and applied through R82 to V32 and through R91to V211. The second effect is the increase in voltage level on thecommon cathode lead caused by the momentary increase in current throughR85, which voltage change makes the several cathodes more with respectto their respective grids. Bot effects tend to inhibit conductionthrough Vfitl and V 31, and also tend to reduce conduction through V23.Conduction through V29 therefore continues to increase, both of theeffects increase in magnitude, and the conductive status is very rapidlyshifted from V28 to V 29. V29 remains in the conductive state untilafter the second sync pulse arrives on lead 161; and in conductingcauses a lower potential level to exist on lead 162 because of theincreased voltage drop across R77. This lowered potential on lead 1&2permits a partial discharge of C79, thereby dropping the negative biason the grid of VSil and preparing the latter tube for capture orassumption of the conductive status upon arrival of the second syncpulse on lead 161. Also the potential drop on lead 162 is employed, in amanner and by means hereinafter discussed, to cause operation of asignal gating relay Rylfl in the second resisitivity signal channel.

The second sync pulse is applied to the grids of all four triodes, bypaths now evident and the same as those followed by the first syncpulse. With V29 conducting, the decreased potential on lead 162 providesa high level of bias on the grid of V28, insuring continuednon-conduction of that triode; and with C71 fully charged the bias onthe grid of V31 is appreciably higher than that on the grid of V3il,capacitor (37% having been partially discharged by the drop on lead1*52. Arrival of the second pulse thus initiates commencement ofconduction through V30. The current through V39 in passing through R87drops the voltage level on lead 163, thus decreasing the voltage levelon grids of both V28 and V29, and concurrently increasing the cathodebias on all four triode cathodes. Again, as formerly, the two concurrenteffects tend to maintain V28 and V31 in the non-conducting state andtend to increase conduction through V3tl and decrease conduction throughV29; and thus the conductive status is shifted from V2 to V3 l. Thelowered potential on lead 163 due to V30 plate current flow through R87permits partial discharge of C71, and thereby prepares V31 for captureof conductive status from V36 upon arrival of the third sync pulse onlead 161. Also, the lowered potential on lea 163 is employed ineffecting signal-gating operation of a relay R 113 interposed in thethird resistivity signal channel. The decreased potential on lead 163,reflected to the grids of V28 and V29, tends to insure continuednon-conduction on the part of those triodes.

In a manner now evident, arrival of the third sync pulse on lead 161initiates capture of the conductive status by V31 from V31), the ensuingtransfer of conduction following the previously enunciated principles.Conduction by V31 lowers the potential on lead 161, and this decrease isemployed to control operation of a signalgatin relay R3 1 interposed inthe fourth resistivity signal channel. After the apparatus is once setin operation, the sequences or repetitive groups of one negative andthree positive sync pulses initiate conduction through V23, V29, V351,and V31 in that order in repetitive cycles, one cycle per commutationcycle in the subsurface apparatus, and in synchronism with concurrentarrival of respective first, second third, and fourth resistivityinformation signals. As previously indicated, the output pulses(negativegoing) of VZEPV 31 created incident to conduction throughrespective tubes of those triodes, are employed for relay control inrespective resistivity signal channels. These output pulses are of acharacter indicated by graphs k, l, m, and n in FIG. 7, and willhereinafter be more fully treated in connection with explanation of thesignal gating operations.

A modified form of the just-described control circuit is adapted toutilize a continuing series of pulses of the same polarity to provideoutput pulses in turn to each of a plurality of lines and performcomputer functions rather than synchronization functions. Thismodification will hereinfater be more fully explained.

Referring specifically to FIG. 2b, it is recalled that the incomingresistivity information pulses arriving on cable eads 2", 3" werechopped at S1 81, transformed at T12, and reconstituted into DC. pulsesat the output contacts SvSlZ of synchronous converter Sv; and that theoutput complex was filtered to clear the 11C. pulse signals of all AC.components by the filter comprising choke C117. In the exemplaryapparatus the D.C. signal pulses are translated onto lead 33 insequences or groups of four. The first pulse of any of these groups isthat representing the short normal resistivity measurement, the secondpulse representing the long normal resistivity measurement, etc, aspreviously made apparent. While all of the resistivity informationsignals are impressed in turn upon lead 33, each is translated therefrominto and through a respective separate signal channel for individualgating, amplification, and translation into an increment of a respectiveresistivity log by a respective recorder means. it will be recalled thatin the subsurface apparatus the four resistivity signals, as picked upat the potential electrodes, were of widely different intensities andwere accordingly subjected to different degrees of attenuation prior topresentation to the single signal amplifying means in that apparatus.The object was, as noted, to provide input signals of intensities withinthe dynamic range that the amplified could accommodate. The degree ofattenuation of signals in any given channel is predetermined, so thatthe mathematical relationship of the attenuated signal to the inputsignal is in each case known. Thus the intensities of the reconstitutedD.C. pulse signals translated onto lead 33 bear known mathetmaticalrclationships to the resistivity measurement values they represent. Onepurpose of providing individual amplifying means in each signal channelin the surface apparatus is to enable the operator to restore the signalin each channel to its true intensity level, or alternatively to apredetermined level bearing a definite known relationship to theoriginal intensity level, for presentation to a respective signalrecorder. Since the four resistivity signal channels fed from lead 33are similar in physical construction and operation, 'difiering only inelectrical values of some components, only the first such channel andits operation will be described in detail. It is to be noted that whileall of the information signals in the form of reconstituted D.C. pulsesare presented in sequence to all of the individual signal channel inputcircuits, each individual signal is, by means of a gating relay,admitted to only its respective individual amplifier.

Signal lead 33 is branched as indicated to provide leads from whichrespective signal pulses of each group are translated into theindividual signal amplifier-recorder circuits. Hie first of thesecircuits, in the signal channel for the first pulse of each group (seethe upper right of FIG. 2b), has a branch from lead 33 over which thesig nal pulse enters through an adjustable resistor Rltlll. Thisresistance is made adjustable to permit of circuit calibration, as willhereinafter be described. The incoming 11C. pulse signal passes toground through a sensitivity-adjusting variable resistor RlltiZ, fromwhich a si nal of selected intensity is derived at slider RlfiZS. Thesignal thence passes through normally closed contacts RyZtlAl of a relayR3 20, and on to a movable contact Ryllm of the aforementioned signalgating relay Ryllii. The latter relay is normally energized (by anodecurrent through a normally conducting relay drive tube Vltll), withmovable contact Ryllm in the upper, open-circuit position, thusmaintaining the first signal channel in a normally open-circuitcondition. The gating performed by relay Rylli is two-fold in purpose.First, the signal translating circuit is closed at only the proper timesto pass the first pulse of each group of reconstituted signal pulses.Secondly, the fall out and pull up of the relay are so regulated orcontrolled that the distorted leading and trailing portions of thesignal pulse are eliminated and only the middle part of each first pulseis passed. The gating and other signal translating circuit of the firstsignal channel operates in the following manner, reference beingdirected also to the wave forms shown in FIG. 7. Relay drive tube V161is normally conducting, and the anode current, indicated by curve (p) ofFIG. 7 energizes the coil of Ryll to maintain the first channel circuitnormally open. Since the negative-going pulses on leads 16%, 162, 163,and 161 occur in series with each following another in time, they may beas indicated by curves k, l, m, and n of FIG. 7. The first of a series,occupying the first period in a signaling cycle and being that producedon lead 160 as tube V23 conducts, appears across ClJZ i and on the gridof drive tube Vltil and causes cut-off in that tube. This may occursubstantially instantaneously with arrival of the control pulse on lead160; however, relay Ryll does not fall out until a short time afterward,due to the time constant of the coil of the relay and capacitor Cllllconnected in series with a resistor (not shown) across the relay coil.The current through Ryll follows curve (p), and is seen to commencedecaying upon arrival of the control pulse and falls to a minimum nearthe middle of the first signal period. Then, due to differentiation ofthe leading edge of the negative-going control pulse by CUB-R163 at theinput to Vltlll, the tube conducts and the relay current rises to amaximum or normal value by the end of the period. During the decay ofrelay current a value is reached, as indicated at point P of curve 1),at which the relay falls out and the signal-translating contacts close.Similarly during the rise of relay current a point P is reached at whichthe relay picks up, opening the signal-trar1slating contacts. Thecircuit constants are so chosen that relay fall-out does not occur untilthe reconstituted DC. pulse signal (811 of curve (0) in MG. 7) hasreached a steady-state value; and also so that as the leading edge ofthe control pulse is diifcrentiated by cltldiiltlfl, relay pick-upoccurs prior to arrival of the trailing end of the signal pulse Sill".In this way the leading end portion of signal pulse Sill" is deniedtranslation through the relay, and the trailing end is likewiseexcluded; and thus only the middle of the signal pulse, as shown hatchedin curve (0) of FIG. 7, is translated through the relay. Thus thefirst-channel signal is such as that indicated at Sz'l." in FIG. 7.

From the preceding it is seen that there is passed through thetemporarily and briefly closed lower contacts of Ryll, a re-formed(shortened) D.C. pulse sample 511' of square wave characteristics, whoseamplitude represents the desired information for an increment of thefirst channel (short normal) resistivity log. This pulse will beseparated in time from the next first channel pulse by a considerableperiod of time during which the second, third and fourth channel signalpulses will be translated into and through respective circuits byoperation of respective gating rcla s R3112, Ryl3, and RyIt l. Since thesignal Sil passed through the gating relay is of brief duration ascompared with the total period of one 4- channel signaling cycle, meansare provided for extending or holding the efiect of the signal so it mayregister on the recorder means for a period equal to the duration of onecomplete signaling cycle. These means comprise an RC network consistingof a capacitor C104 and a resistor RT M connected as indicated in theinput circuit of a modified push-pull amplifier comprising the twotriode amplifier tubes, VltlZ, V193. A common cathode resistor RltlSinterconnects the amplifier cathodes and ground. By this circuitry,push-pull amplifier operation is obtained without the complexity ofordinary push-pull circuitry, and the dynamic range of the amplifier isdoubled. It is to be noted that the amplifier is a DC. amplifier, onlypositive-going DC. pulses being applied to the input. The signalpersistence circuit comprising R194 and C164, which desirably mustmaintain the signal in substantially undiminished intensity during aperiod intervening termination of the incoming signal sample and thenext fall out of Ryli, is composed of elements providing a large timeconstant; for example, ten seconds when the complete signaling cycle issecond long. Thus when a DC. signal pulse sample is passed by relay R311, the grid of Vltlf'l is brought substantially to the potential levelof the signal pulse and is maintained at substantially that level by theR-C pulse persistence network until passage of the next first-channelsignal by relay Ryll. The grid and R-C circuit potential level is, ofcourse, reset by each admitted pulse, being raised if the first-channelsignal intensity is in the increasing direction and reduced if thesignal strength is decreasing.

As a result of the admission of a selected portion or sample of a DC.pulse signal through relay Ryll, there is thus produced at the output ofamplifiers V102V103 a continuous signal whose intensity is re-set onceeach signaling cycle (once during each second in the example). Thiscontinuous signal, developed as the difference in potential drops acrossanode resistors R105 R106, is utilized by conventionalgalvanometer-recorder means herein represented by RG1, to produce acontinu ous graph or log of the discrete information samples secured bythe first resistivity information channel in the subsurface apparatus.The graph or log is produced in a known manner to show the resistivitymeasurement values related to the position or depth in the borehole atwhich the respective measurement values were obtained.

In previous paragraphs it has been explained how the first output pulseof the sync or control circuit, which oneness pulse appeared on leadlldtl, caused timed closure and reopening of normally open contacts ofrelay Ryll to select and pass a sample portion of the first DC. signalpulse contemporaneously arriving over lead 3 3. In a similar manner, thesecond output pulse of the sync or control circuit (produced. byconduction through V29 and appearing on lead 162-), causes fall-out andpick-up operation of second channel gating relay R3 12, to in a similaroperation pass a middle portion Sill of the concurrently arriving secondDC. signal pulse 512 into a similar signal-persistence and amplifiercircuit which feeds a second galvanornetcr-recorder unit RG2. The latterin a similar fashion produces the second (long normal) resistivity log.In manner now evident and by similar means depicted, the third (si ortlateral) resistivity signal S13 is modified by gating and holding andpassed to a respective galvanometer-ecorder unit tGES by operation ofgating relay Rylil in response to a pulse produced on lead I163 incidentto conductivity status shift from V29 to V34 Similarly the fourthresistivity signal Sid" is modified and fed to a galvanorneter-recorderunit RG4 by fall-out operation of gating relay R3 14 in response to apulse created on lead lei incident to conduction through V31 it will berecalled that the original resistivity signals were picked up withrespective intensities which dillered considerably, and that the signalswere attenuated to diiferent extents to accommodate the signals to therange of a single signal amplifier in the subsurface apparatus. As acons-2- quence, the signals as presented to the galvanometerrecorderunits in the surface apparatus are related in intensity to the originalsignals by predetermined known relationships determined by therespective attenuations, amplifications, and circuit element values. Byproper assignment of values to transverse scalar divisions on therecording mediums or papers in the respective galvanometer-recordcrmeans, the eilects of the dillerent degrees of signal attenuation in thesubsurface signal channels may easily be accounted for, whereby thegraphs or logs present accurate portrayals of the original signalspicked up at the respective electrode pairs. Accommodation orcompensation of the different attenuations applied to the signals inrespective channels may also be effected in whole, or in part, bycorrespondingly ditlerent ratios or degrees of amplification,attenuation, etc., in the surface apparatus. For example, eachgalvanometerrecorder unit may, in accord with conventional practice, beprovided with variable attenuating and/or amplifying input circuits.

Power Supply A regulated supply of power and potential for indicatedcomponents of the surface apparatus is provided. Power is derived fromsuitable AC. mains, such as the 115 v. AC. mains depicted in the upperportion of FIG. 2b, by a transformer Till which has several secondarywin ings as indicated. One of the secondary windings is centcr-tappedand supplies anode potential for a full-wave rectifier tube Vlt d whosecathodes are heated by power derived as indicated from another secondaryWinding. Filtered DC. potential is supplied from Vltld to a lead litl,and this potential is employed for anode supply for VZii as indicated,and for a regulated power supply unit depicted within the dash-linerectangle 171 in FIG. 2b. This unit includes tubes Vldii, Vim, Vidal,Vltii", and V119, and provides a regulation better than 0.1% and along-term output consistency of 0.1%. The unit as diagrammaticallydepicted is or may be of conventional design; and may be replaced by anyconventional power supply unit of comparable characteristics. The unitprovides, through the previously mentioned B-lsupply lead 155, constantanode potential to the control-circuit elernents, the relay controltubes Vltll, etc., and to the signal amplifier circuits, all byconnections as shown in FIGS. Za-Zb. Also from lead 155 there issupplied potential for circuit calibrating means for the NP. andresistivity recorder circuits, as will be in greater detail explainedmar (Ir s? hereinafter in connection with a description of apparatuscalibration means and procedure. Transformer TM? is provided with anauxiliary secondary T2985 which provides, through a normally opencalibration switch SW0, power to energize the coil or" theaforementioned normally relaxed relay R324 (FIG. 2a), and the coil ofrelay RyZll (FIG. 2b) which operates contacts RyZilAl, etc. Switch Swcis closed only when it is desired to calibrate certain components of thesubsurface and surface apparatuses.

Calibration Means and Procedure Referring of FIG. 25, it is seen thatclosure of switch SW0 permits current supplied from tranfsormer T29 toflow through a lead 1W5 and through the coil of relay lly i (FIG. 2a),whereby the latter pulls up and connects input signal leads 2"-3 acrossa selected portion of a variable resistance RLIZll inserted in B-[- leadas indicated. This causes application or" a selected DC. potential (cg.1.0 v.) to signal leads 23l in lieu of the normal DC. pulse signals, thevalue of the potential ap plied being indicated by a voltmeter V whichmay ternporarily be connected across leads ZW-fi. Closure of switch Swcalso causes relay RyZtl to pull up, hereby contact RyZtlAl is raised toeliminate sensitivity-varying attenuation resistor Rid from the signalinput to the first channel amplifier, and leaving only calibrationresistor Rid in that circuit. Rfltll. is then so adjusted as to produce, say, full scale deflection. of the galvanometer unit in RG1 (theinput signal potential on 21"-3" having been adjusted to a standard,such as, for example, 1.0 v., as stated, and the galvanometer havingbeen standardized at, for example, 25 microamperes for full scaledeflection), whereby the first signal channel is calibrated. V ariationof resistor R162 is employed to cause recording at a sensitivity valuebelow the value prescribed by a selected one of the resistors in Rail(FIG. 3b) of the sub surface apparatus. The sensitivity adjustment isnot made during the calibration procedure, however. Adjustment, orcalibration, of the circuit for second channel signals for recording onRG2 is made in the same general manner as just indicated for the firstchannel; and similarly for the respective signal channels leading to RG3and RG4, it being noted that RyZ-fl operates appropriate contacts in allor" the resistivity signal channels.

Pull-up of relay Ry l (FIG. 2a) in response to closure of switch SW0(FIG. 212), causes disconnection of the NP. recorder and associatedinput circuitry from between lead 1 and surface ground G(Szsr) atcontacts Ry ia, and connection of the recorder NEG. between lead S and apoint on a variable resistor R1211 connected in lead 15?. Thus the NP.recorder input circuit has impressed thereon a DC. potential derivedfrom a portion of the 1R drop across R121. This potential is measured bymeasuring the current produced thereby through a resistor R122 of knownlarge value, and adjusted by varying Rlilll to produce a current of avalue known to be required for full scale deflection of the recordergalvanometer; for example, 25 us. In the event the galvanometcr does notregister exactly full scale deflection, it is brought to that state byvarying the slider on a variable resistor R41. Prior to operating thelogging tool within. a bore hole, the proper potential for full scaledeflecting may be applied between subsurface apparatus electrode Eb andthe wire connection to the surface ground electrode, with relay Rydinunenergized state. In both cases, the movable contact of a potentiometerP01! is assumed to have been set at the position corresponding to thedesired sensitivity at which the NP. log is to be made.

There is provided in a panel unit Pan (see the lower left of FIG. 2a),switching and power means whereby an operator at the surface apparatusmay control certain operations of components of the subsurface apparatusand thereby change the sensitivity settings of the four resistivitychannels, may switch circuits from an operating condition to anindicating condition and vice versa, and may determine conditions ofcircuits and positions of switches. Referring to FIG. 8, in which theessential components of unit Pan are diagrammatically displayed inschematic form in a dash-line enclosure labeled Pan, it is noted thattwo sources of direct current, such as batteries B]; and B2, areconnected to the center (input) terminals of respective polarityreversing switches SE1 and 8B2, and that the output terminals of SB].are connected to supply battery current of either polarity to sheathconductor S and conductor 3 of the cable (via S and 3') while the outputterminals of S32 are connected to supply current of either polarity toconductors Z and 3 (via 2 and 3). A short-circuiting switch SSC isconnected to permit connection of lead l to lead S, and a switch SOCpermits opening and closing signal leads 2-3.

When it is desired to change the sensitivity-setting of the resistivitysignal channels at Rel, R02, Rcfl, and R04 in the logging tool, switchSSC is closed, thereby shortcircuiting the constant-current supplysystem output fiov ing through S'1' and produced at the secondary of Tt. Thus the alternating current low through cable conductors 1 and S isterminated, and relay RyZJ (FIG. 3a) in the subsurface apparatus istie-energized and falls out, connecting cable conductor 2 to a lead 132and conductor 3 to a lead 183. Switch SOC is opened to break the signalcircuit into the signal-utilizing components of the surface apparatus.Switch SBZ of the panel unit is then moved to the right to applypolarity energy from E2 to conductor 2 and polarity to conductor 3.Current then flows down conductor 2 into lead 132, through a rectifierRecS and the next to top deck of rotary switch Rsl into a lead 1&5,through a resistor 1205c of bank R05, into lead 186 and into lead toreturn on cable conductor 3. Resistors Roda, R051), etc, are ofprecision type of known values, hence by reading ammeter All in thepanel unit Pan the operator may readily determine by current magnitudethat the units of rotary switch RS1 are in the X position as shown inFIG. 3b. In a similar fashion the placement of the sets of contacts ofRsl in Y position can be determined, since the Rsll in that position theindicating current flow is through resistor Rc5c rather than throughR052 as before. And the Z position of Rsl. is detected or indicated by acurrent of the same direction that flows through resistor RcSa ratherthan through Rc5e or R050. Hence with panel switch SE2 thrown to theright, the positional status of RS1 is readily determinable. Reverseoperation of panel switch SE2 reverses the podarity of voltage appliedto conductors 2 and 3, and current then fiows in the reverse direction,down conductor 3 into lead 133, through lead the, resistor R050 of bankRcS, through the movable contact of the top deck of a multi-deck rotarystep-by-step switch RsZ, rectifier Rec 4-, and lead 182 to return lead2. Resistor RcSa is of value such that the operator by reading ammeterA2 may determine that switch is in the a position. Similarly, if RS2 isin the 1: position, the current flow will be from lead 136 throughRat-3b and return via Read; and if RS2 is in the 0 position the currentflow will be through R050, etc. Thus with switch SOC open and switch SBZto the left, the positional status switch RsZ may be readily determined.

Now, with switches SOtI and S82 open, closure of panel switch SE1 to theright will apply polarity DC from battery B1 to lead 3' and polarity tolead S, to cause current to flow down through conductor 3, lower movablecontact of de-energized relay R 23, lead 133, rectifier Reel, steppingcoil RslC of multi-deck rotary switch RS1, to ground lead Gr and returnby way of sheath S. This current flow will energize RslC and therebyadvance switch Rs]; one step, from the X position to the Y position.Opening of panel switch S31 and subsequent reclosure thereof will againadvance switch Rsl one step, from Y position to Z position; andadditional reopening and reclosure of S31 as above indicated will stepRS1 to the original, or X position. Thus the positional status of R51may be readily and rapidly changed at the will of the operator.

Further, with switches SOC and S82 open as indicated in the precedingparagraph, closure of switch SE1 to the left causes current flow in theopposite direction, down through sheath S, ground lead Gr, throughstepping coil RstZC of five-deck rotary stepping switch RsZ, lead 183and return via conductor 3. This causes advance of R92 from the a to t.e b position; and repetitive opening and closing of S131 to the leftwill in evident manner cause stepping of Rsil through the c, "d, e, andf positions and back to the a position. Thus RS2 may be set, or re-set,to any of the mentioned positions by appropriate opening of switch SOCand operations of panel switch SE1. As is evident, the operator may,after repositioning either or both of switches RS1 and RsZ, check theresults of the operation by the previously described switch-positionindicating procedures.

Switch RsZ may be actuated to set or change the setting of thesensitivity at which the resistivity logs are made. For example, theresistors corresponding to the "a position in banks R01, R02, R03, andRed, may be of selected values to provide IO-Ohmmeter sensitivity; withthe resistors corresponding to the b positions selected for ZG-ohmmeterlogs, etc. In the exemplary embodimeut of apparatus the six resistors inany of the re spective banks provide for logs of 10, 20, 30, 40, 60, andohmmeter sensitivities, but it is evident that other sensitivitygradations may be accommodated by appropriate selection of resistorvalues. As before noted, curves of any sensitivity intermediate thosevalues provided for by the resistors of Rel, etc., may be run bysuitable change of the slider on resistor R161 in the first signalchannel, etc. Further, it is evident that by employing more switchstations in each bank or deck of Rafi, and corresponding additionalresistors in the banks, additional sensitivity ranges may readily beaccommodated by the system.

Referring again now to the four-triode control circuit comprising V28,V29, Vdtl, and V 31, and to the previous description of its operation,it is thought to be evident that the circuit is not limited to fourtriodes and associate circuits. Additional triodes, each provided with aresistance-capacitance network in its grid circuit connected to bepartially discharged by conduction through a preceding triode, andhaving a respective anode load resistor and output line, could beemployed with an input pulse group composed of one pulse and one pulseper triode following the first triode. That is, it a series of N triodeswere provided, each but the first having the resistance-capacitance netand the series were fed a negative pulse followed by N-1 positivepulses, an output pulse could be supplied in each of N output lines forsynchronizing controlling actions in N controllable circuits. Eachseries of input pulses comprising a re-sctting pulse (in this case thenegative pulse), cyclical synchronizing action would be assured.

Additionally it is thought to be evident that if the first triode of aseries of N triodes were provided with an R-C network in its input gridcircuit, and each of the remaining triodes were similarly equipped andall were otherwise the same for each triode as in the case of triodesV28-V33l, the control circuit would operate as a ring circuit of N linksupon being fed a continuing succession or series of only pulses. Forexample, if a capacitor of proper value were shunted across resistor R73in the input to V28, as indicated by C68 in FIG. 9, it would chargeduring non-conductance of that triode, would partially discharge due toV3]. (or the last triode in the ring) conducting, and would seizeconductive status from the latter tube upon receipt of a pulse, in themannot explained in connection with conductive status seizure by V29,V30, etc. Obviously only pulses would be necessary for causingsuccessive conduction in the order: V28, V29, V3t V31, V28, V29, V30,etc. And in this

1. A CIRCUIT FOR GENERATING A PLURALITY OF ELECTRICAL PULSES HAVINGSTEEP WAVE FRONTS AND FAST RISE TIMES IN RESPONSE TO INPUT WAVE FORMSHAVING POSITIVE AND NEGATIVE POLARITY AND HAVING A WIDE RANGE OF SHAPESAND AMPLITUDES COMPRISING, IN COMBINATION, FIRST AND SECOND VARIABLERESISTIVE MEANS, MEANS FOR NORMALLY MAINTAINING SAID FIRST VARIABLERESISTIVE MEANS AT A RELATIVELY HIGH EFFECTIVE RESISTANCE AND SAIDSECOND VARIABLE RESISTIVE MEANS AT A RELATIVELY LOW EFFECTIVERESISTANCE, SAID FIRST MEANS RESPONSIVE TO THE POSITIVE POLARITY OF SAIDINPUT WAVE FORM TO REDUCE ITS NORMALLY HIGH RESISTANCE AND SAID SECONDMEANS RESPONSIVE TO THE NEGATIVE POLARITY OF SAID INPUT WAVE FORM TOINCREASE ITS NORMALLY LOW RESISTANCE, AND A PAIR OF SUBSTANTIALLYINSTANTANEOUS SWITCHING CIRCUITS RESPECTIVELY ACTUATED BY CHANGES INSAID FIRST AND SECOND VARIABLE RESISTIVE MEANS TO GENERATE PULSES HAVINGSTEEP WAVE FRONTS AND FAST RISE TIMES INDEPENDENT OF THE SHAPE OF SAIDINPUT WAVE FORM.